The present invention relates to atmospheric sciences, and, in particular, relates to an apparatus and a method of determining dew point temperature and/or water vapor pressure, for example.
Radiosonde humidity measurements are routinely made with transducers which respond to the relative humidity of the air, i.e., carbon element, humicap sensor, lithium chloride, hair, goldbeaters skin. All are designed to provide a variation in some electrical quantity such as resistance or capacitance or impedance with changes in relative humidity. The value of this electrical property is then sensed and telemetered to the ground where it is used to compute the relative humidity. All of these devices exhibit some temperature dependency, that is the relationship of the electrical quantity used to relative humidity is not one curve, but a family of curves, one for each temperature.
The most commonly used sensor in the United States is the carbon element. In radiosonde applications the carbon element is located in a duct which serves the dual purpose of shielding the element from rain and from insolation. Since the carbon element responds to relative humidity (RH), it is necessary to know the temperature of the air to calculate other measures of humidity, e.g., water vapor pressure, dew/frost point temperature, specific humidity, absolute humidity and mixing ratio. In some of these, the pressure of the air is also required for calculation. In attempting to measure the relative humidity of the air, the defining temperature is the temperature of the surface of the sensor which thermodynamic considerations dictate to be identical with the air temperature immediately in contact with the sensor. This surface temperature is in general distinct from the free air temperature, i.e., the temperature of the air before it comes in contact with any part of the radiosonde or the sensor itself. In addition, any error in determining the free air temperature can introduce error in the humidity terms computed from the relative humidity measurement.
The most basic and the largest source of error in the radiosonde carbon humidity type of measurement is caused by the temperature difference between the surface of the carbon element and the free air temperature. When a parcel of air comes in contact with the sensor the temperature of the parcel is changed from its free air value to the sensor surface value. This results in the relative humidity of that parcel being changed and it is this modified value of relative humidity which is then sensed by the carbon element. This error results in related errors in any of the other measures of humidity calculated using this value.
Besides the temperature effects there are two other characteristics which result in significant errors: the element has poor sensitivity at low relative humidities (RH&lt;25%) and its response time characteristic degrades markedly at low temperatures. Because of the low sensitivity at low RH, it is standard practice not to report humidities below 20% RH on synoptic radiosondes. Because of the response time degradation, the depiction of humidity featureas with vertical scales of less than 400 m will be very limited at temperatures of -20.degree. C. or lower. All three of the error producing characteristics can severely affect the measurement accuracy at any altitude but their combined influence is most often felt at higher altitudes (&gt;4 km).
The following patents are incorporated by reference as to their teachings on humidity detection, etc.: U.S. Pat. Nos. 4,911,357; 4,801,211; 4,080,564; 4,793,182; and 4,793,181.
In recent years there has been an increasing requirement for more accurate humidity measurements. These requirements come from satellite applications that require improved humidity data both to calibrate and to validate their systems performance for new atmospheric models that are sensitive to middle and upper troposphere moisture, and from military applications with the increased emphasis on electro-optical systems. Not only is there an increased need for accuracy in general but many of the new requirements are for increased accuracy at the higher altitudes where the current measurements are most deficient.